This invention relates to radio receivers for amplitude modulated (AM) signals and more particularly to AM radio receivers having improved selectivity.
The selectivity of a radio receiver is the characteristic of the receiver which determines the extent to which the receiver is capable of distinguishing between a signal, that is a signal in a channel to which the radio receiver is tuned, and other signals, that is, signals in channels to which the receiver is not tuned, usually signals in channels adjacent to the tuned channel.
The radio frequency spectrum for aircraft radio communications receivers is comprised of a plurality of individual contiguous communications channels, each of which spans an assigned frequency band. In a crowded radio environment, that is, where many of the channels are occupied by active transmitters, it is important that receivers with good selectivity be used to avoid interference from adjacent channel signals into the channel to which a particular receiver is tuned.
The conventional approaches to obtaining desired selectivity in AM radio receivers include the use of multiple tuned IF or RF circuits or the use of a single, multiple element crystal or ceramic filter to provide the required frequency selection. However, as channel spacings in the AM radio frequency spectrum decrease in response to the demand to increase the number of channels in a given spectrum band, the selectivity requirements to which receivers are designed become increasingly more difficult to achieve with the conventional design approaches. More specifically, the conventional elements used to provide selectivity have a response versus frequency characteristic which includes a pass-band at the desired channel and trailing skirts at either side thereof. When the next lower channel and next higher channel are moved closer to the desired channel to make more room in the AM spectrum for additional channels, these adjacent channels move into the skirts of the selectivity characteristic. This, of course, will cause adjacent channel interference in the tuned or desired channel. It is thus important, to avoid such adjacent channel interference, to redesign the selectivity element to provide more sharply trailing skirts in its selectivity characteristic.